1. Field of the Invention
The present invention relates to a method for preparing a diene elastomer, such as a butadiene homopolymer or copolymer. The invention especially applies to the production of a polybutadiene with a high level of cis-1,4 linkages that has both a Mooney viscosity greater than or equal to 40 and a reduced polydispersity index.
2. Description of the Related Art
In order to prepare butadiene homopolymers or copolymers having a high level of cis-1,4 linkages, it is known to use catalyst systems based on:                a salt of rare-earth metal(s) in solution in a hydrocarbon-based solvent;        an alkylating agent of this salt formed from an alkylaluminum; and        an alkylaluminum halide.        
Patent document EP-B-207 558 teaches, in order to obtain a butadiene homopolymer or copolymer having a Mooney viscosity that is more or less independent of the degree of conversion of the monomers:                initiating the polymerization reaction with a catalyst system comprising:                    a) a neodymium compound chosen from neodymium oxide, and neodymium alkoxides, phenates and carboxylates;            b) an organic compound having a hydroxyl or carboxyl group;            c) a non-halogenated organometallic aluminum compound; and            d) a halogenated compound chosen from secondary or tertiary alkyl halides, organic acid halides, metal and organometallic halides, hydrogen halide acids and halogens,                        so that the aluminum/neodymium molar ratio in this catalyst system varies from 10 to 30; and        adding in continuous or batch mode, after the start of the polymerization reaction and over a time period at least equal to half of the total duration of this reaction, a given amount of this compound c) until an aluminum/neodymium molar ratio ranging from 20 to 80 is obtained.        
As indicated in the embodiment examples of this document, the purpose of this addition of the organoaluminum compound after the initiation of the polymerization reaction is to obtain Mooney viscosities ML (1+4) that are always substantially below 40, in comparison with the Mooney viscosities that are obtained by adding this compound at the same time as the catalyst system, which does not give these polybutadienes that are well suited for use in tire cover treads.
Patent documents WO-A-02/38636 and WO-A-03/097708 in the name of the Applicants teach, for obtaining polybutadienes, the use of a “preformed” type catalyst system based on at least:                one preforming conjugated diene, such as butadiene;        one organic phosphoric acid salt of one or more rare-earth metal(s), which salt is in suspension in at least one saturated and aliphatic or alicyclic, inert hydrocarbon-based solvent;        an alkylating agent formed from an alkylaluminum of formula AlR3 or HAlR2, and        a halogen donor that belongs to the family of alkylaluminum halides excluding alkylaluminum sesquihalides.        
The polybutadienes obtained using this catalyst system have, in particular, a polydispersity index less than 2.1 and a Mooney viscosity ML (1+4) at 100° C. greater than or equal to 40. These combined characteristics make these polybutadienes well suited for use in tire cover treads.